Universal control for implementing quantum gates

ABSTRACT

Methods, systems, and apparatus for implementing a unitary quantum gate on one or more qubits. In one aspect, a method includes the actions designing a control pulse for the unitary quantum gate, comprising: defining a universal quantum control cost function, wherein the control cost function comprises a qubit leakage penalty term representing i) coherent qubit leakage, and ii) incoherent qubit leakage across all frequency components during a time dependent Hamiltonian evolution that realizes the unitary quantum gate; adjusting parameters of the time dependent Hamiltonian evolution to vary a control cost according to the control cost function such that leakage errors are reduced; generating the control pulse using the adjusted parameters; and applying the control pulse to the one or more qubits to implement the unitary quantum gate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of, and claims priorityto, U.S. patent application Ser. No. 16/961,388, filed on Jul. 10, 2020,which application is a National Stage Application under 35 U.S.C. § 371and claims the benefit of International Application No.PCT/US2018/016225, filed Jan. 31, 20218. The disclosure of each of theforegoing applications is incorporated herein by reference.

BACKGROUND

This specification relates to quantum computing.

Large-scale quantum computers have the potential to provide fastsolutions to certain classes of difficult problems. In gate modelquantum computers, computational tasks are performed by implementingsequences of universal quantum gates, each of which specifies atrajectory of quantum computer's evolution. The faster quantum gateexecutes, the more computational capacity a given quantum devicepossesses.

SUMMARY

This specification describes methods and systems for universal controlwhen implementing arbitrary quantum logic gates.

In general, one innovative aspect of the subject matter described inthis specification can be implemented in a method for implementing aunitary quantum gate on one or more qubits, the method including theactions of: designing a control pulse for the unitary quantum gate,comprising: defining a universal quantum control cost function, whereinthe control cost function comprises a qubit leakage penalty termrepresenting i) coherent qubit leakage, and ii) incoherent qubit leakageacross all frequency components during a time dependent Hamiltonianevolution that realizes the unitary quantum gate; adjusting parametersof the time dependent Hamiltonian evolution to vary a control costaccording to the control cost function such that leakage errors arereduced; generating the control pulse using the adjusted parameters; andapplying the control pulse to the one or more qubits to implement theunitary quantum gate.

Other implementations of this aspect include corresponding classical orquantum computer systems, apparatus, and computer programs recorded onone or more computer storage devices, each configured to perform theactions of the methods. A system of one or more computers can beconfigured to perform particular operations or actions by virtue ofhaving software, firmware, hardware, or a combination thereof installedon the system that in operation causes or cause the system to performthe actions. One or more computer programs can be configured to performparticular operations or actions by virtue of including instructionsthat, when executed by data processing apparatus, cause the apparatus toperform the actions.

The foregoing and other implementations can each optionally include oneor more of the following features, alone or in combination. In someimplementations the universal control cost function further comprises acontrol constraint penalty term.

In some implementations the universal control cost function furthercomprises a total runtime penalty term.

In some implementations adjusting parameters of the time dependentHamiltonian evolution comprises adjusting parameters of the timedependent Hamiltonian evolution to vary the control cost according tothe control cost function such that total quantum gate runtime isreduced.

In some implementations the universal control cost function furthercomprises a gate fidelity penalty term.

In some implementations adjusting parameters of the time dependentHamiltonian evolution comprises adjusting parameters of the timedependent Hamiltonian evolution to vary the control cost according tothe control cost function such that quantum gate fidelity is increased.

In some implementations defining the universal quantum control costfunction comprises: generating a block-off-diagonal component of aneffective Hamiltonian with direct coupling leakage errors suppressed tosecond any given order; determining a coherent qubit leakage penaltysub-term using the generated block off-diagonal component of theeffective Hamiltonian; determining an incoherent qubit leakage penaltysub-term using the generated block off-diagonal component of theeffective Hamiltonian; and defining the qubit leakage penalty term asthe sum of the coherent qubit leakage penalty sub-term and theincoherent qubit leakage penalty sub-term of both on-resonant andoff-resonant frequency components.

In some implementations generating the block-off-diagonal component ofan effective Hamiltonian comprises generalizing the time-dependentSchrieffer-Wolff transformation to suppress direct coupling qubitleakage to second any given order, comprising: separating a Hamiltoniancharacterizing the one or more qubits into multiple components, thecomponents comprising a first component representing time-dependentcoupling between different qubit energy subspaces, wherein theHamiltonian is expressed in terms of multiple basis states; defining aneffective Hamiltonian by performing a Schrieffer-Wolff transformation torotate the Hamiltonian basis states, the effective Hamiltoniancomprising an anti-Hermitian rotation operator; and determining a secondgiven order solution of the rotation operator to generate ablock-off-diagonal component of the effective Hamiltonian with directcoupling leakage errors suppressed to second the given order, optionallywherein the given order is second order.

In some implementations the multiple components further comprise asecond component representing a time-independent part of the Hamiltonianand a third component representing time-dependent coupling within eachqubit energy subspace.

In some implementations each qubit energy subspace is separated fromothers by a sufficiently large energy gap.

In some implementations the determined second given order solution ofthe rotation operator comprises a perturbative solution.

In some implementations determining a coherent qubit leakage penaltysub-term using the generated block off-diagonal component of theeffective Hamiltonian comprises: determining a sum of the amplitudes ofall qubit states outside the computational energy subspace due to theblock-off-diagonal component of the effective Hamiltonian.

In some implementations the coherent qubit leakage penalty sub-term isbounded above by a Frobenius norm of the block off-diagonal component ofthe effective Hamiltonian under an off-resonant assumption.

In some implementations determining an incoherent qubit leakage penaltysub-term using the generated block off-diagonal component of theeffective Hamiltonian comprises: generalizing the applicability of theadiabatic theorem from pure states to energy subspaces, enablingtime-dependent Hamiltonian evolution to occur within and betweendifferent energy subspaces; and determining the incoherent qubit leakagepenalty sub-term using the and the generalized adiabatic theorem.

In some implementations the coherent qubit leakage penalty sub-termdominates in an off-resonant frequency regime and the incoherent qubitleakage penalty sub-term dominates in an on-resonant frequency regime.

In some implementations incoherent leakage comprises leakage resultingfrom violation of adiabaticity during a time dependent Hamiltonianevolution.

In some implementations coherent leakage comprises leakage resultingfrom direct coupling between a qubit's computational energy states andhigher energy states.

In some implementations both coherent leakage and incoherent leakagecomprise on-resonant frequency and off-resonant frequency leakage.

In some implementations adjusting parameters of the time dependentHamiltonian evolution to vary a control cost according to the controlcost function such that leakage errors are reduced comprises applyingoptimization techniques.

In some implementations the one or more qubits comprise superconductingqubits.

The subject matter described in this specification can be implemented inparticular ways so as to realize one or more of the followingadvantages.

A system implementing quantum gates with universal control, as describedin this specification, may improve the functionality and computationalefficiency of a quantum computing device or hybrid classical-quantumcomputing device. For example, a quantum computing device performing thetechniques described herein may implement quantum gates with reducederrors and required runtime, as well as increased quantum gate fidelity.As another example, an optimal balance between quantum gate runtime andfidelity may be determined. In addition, by implementing high fidelityquantum gates in a shortened gate runtime, near-term quantum computingdevices may be used to solve hard problems outside of a classicalcomputer's reach. Furthermore, the universality of the techniquesdescribed in this specification provides improved controllability of aquantum device.

A system implementing quantum gates with universal control, as describedin this specification, may require less computational resources and timeto implement high fidelity quantum gates. For example, designing auniversal cost function that penalizes the form in which control pulsesare used to realize quantum gates can greatly simplify an associatedquantum control optimization procedure since minimizing a scalarfunction requires less computational resources and is a faster processthan directly optimizing a high dimensional unitary evolution governedby the Schrödinger equation of a time-dependent quantum system and itsenvironment.

A system implementing quantum gates with universal control, as describedin this specification, may suppress all kinds of leakage errors acrossdifferent frequency regimes during a generic time-dependent Hamiltonianevolution and is not restricted to suppressing leakage errors from asingle source. Furthermore, all kinds of leakage errors are suppressedwithout requiring hard constraints on allowable forms of Hamiltonianmodulation that impair the universality of the quantum control.

A system implementing quantum gates with universal control, as describedin this specification, may implement arbitrary unitary single andmulti-qubit gates.

For convenience, the techniques described in this specification aredescribed as implementing a single quantum gate on one or more qubits.However, the applicability of the described system and techniques isfully scalable and may be extended to the implementation of sequences ofquantum gates, where respective controls used to implement the sequenceof gates can be merged into a single control, providing a speed up incomputation time whilst increasing gate sequence fidelity.

The techniques described in this specification include a generalizationof the adiabatic theorem from pure states to energy subspaces, allowingfor time dependent Hamiltonian evolution to occur both within andbetween different subspaces. The generalized adiabatic theorem may havevarious applications that require the calculation of the populationtransfer between different subspaces where such generalized adabaticityapplies, including leakage estimation, tunneling rate calculation andadiabatic quantum algorithm designs.

The details of one or more implementations of the subject matter of thisspecification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example device for implementing high fidelity quantumgates.

FIG. 2 is a flow diagram of an example process for implementing aunitary quantum gate on one or more qubits.

FIG. 3 is a flow diagram of an example process for defining a qubitleakage penalty term in a universal quantum control cost function.

FIG. 4 is a flow diagram of an example process generalizing thetime-dependent Schrieffer-Wolff transformation to suppress directcoupling qubit leakage to any given order.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Due to prevalent noise and dissipation, existing quantum computers canoperate for only a short amount of time before uncorrectable errors setin, destroying the embedded quantum information. In gate-model quantumcomputation, computational tasks are carried out by a sequence ofuniversal quantum gates, each of which specifies a trajectory of thequantum computer's evolution. Generally, the faster a quantum gate orsequence of quantum gates executes, the more computational capacity thequantum computer possesses.

A major obstacle preventing shorter quantum gate execution time isleakage errors. Leakage errors may be defined as leakage of quantuminformation encoded in the state of a qubit from a predefinedcomputational subspace into a higher dimensional sub space.

There are two distinct sources of leakage errors: coherent leakage errorand incoherent leakage error. Coherent leakage error results from thedirect coupling between a qubit's computational subspace and higherdimensional subspace. Incoherent leakage error results from modulationof the system Hamiltonian in a time-interval shorter than allowed by thecondition for adiabaticity. For example, superconducting qubitarchitectures typically implement quantum gates via continuousmodulation of a corresponding system Hamiltonian. An intuitive approachto realize a faster quantum gate is to increase the modulation of thequantum system. However, increased modulation results in more energybeing pumped into the quantum computer which will more likely excite thequantum state of the quantum system to higher energy states that areoutside of the predefined computational subspace, resulting in theintroduction of incoherent leakage errors. Both sources of leakage errorcan be further categorized into on-resonant and off-resonant frequencycontributions depending on whether its frequency component equals theenergy gap between a qubit state and a higher excited state(on-resonant) or not (off-resonant).

Conventional techniques for characterizing and eliminating leakageerrors consider the two different sources of leakage errors separately.For example, known techniques for characterizing and eliminatingcoherent and incoherent leakage errors include placing hard constraintson allowable forms of the Hamiltonian modulation. To avoid on-resonantincoherent leakage, a specific functional basis may be chosen for thecontrol pulse that implements a quantum gate to zero out the unwantedfrequency components. To avoid direct coupling leakage, microwavecontrols that couple a computational subspace with neighboring higherenergy levels may be turned off when implementing a two-qubit entanglinggate. These techniques, however, impair the universality of the quantumcontrol: a single controlled time-dependent Hamiltonian evolution can nolonger simulate arbitrary unitary gates. Another known technique forsuppressing coherent or incoherent leakage errors is based on aperturbative diagonalization of the system Hamiltonian throughapplication of the Schrieffer-Wolff transformation (SWT) ortime-dependent SWT (TSWT).

Leakage errors are difficult to eliminate not only because of thefundamental trade-off between quantum gate fidelity and quantumoperation run time, but also due to the current lack of a unifiedmeasure that accounts for total leakage error resulting from bothsources. Without such a universal measure of total leakage error, it ischallenging to directly design an optimal quantum control scheme forhigh fidelity fast quantum gates.

This specification describes methods and systems for reducing quantumgate time and leakage errors from both sources across differentfrequency regimes whilst increasing quantum gate fidelity. Inparticular, a universal control cost function that puts soft constraintson total leakage errors, quantum gate time and quantum gate infidelityin form of penalty terms, whilst guaranteeing the universality ofquantum controls for simulating any unitary gate is described.

The universal control cost function bounds total leakage error. A totalleakage bound, referred to herein as time dependent Schrieffer-Wolfftransformation based leakage bound (TSWTLB), is formulated through thedevelopment of a generalized time-dependent Schrieffer-Wolfftransformation (TSWT). The TSWTLB takes advantage of beneficial virtualtransitions between the computational subspace and the unwanted higherenergy subspaces while providing an upper bound on both direct coupling(coherent) and non-adiabatic (incoherent) leakage errors caused by boththe on-resonant and off-resonant couplings throughout time-dependentHamiltonian evolution.

To derive this leakage bound, a general time-dependent Schrieffer-Wolfftransformation (TSWT) is formulated and its solution given up to anygiven order, e.g., second order. The generalized TSWT provides a rotatedbasis supported in a higher dimensional subspace where direct couplingleakage errors are suppressed to the given order. This rotated basiscoincides with the qubit basis at the beginning and the end of quantumcontrol procedure to enable convenient state initialization and readout.A first leakage bound corresponding to direct coupling leakage isformulated using the generalized TSWT.

A second leakage bound is formulated through a generalization of theadiabatic theorem from pure states to energy subspaces. This allows fortime-dependent Hamiltonian evolution to occur both within and betweendifferent subspaces. The generalized adiabatic theorem provides an upperbound on the non-adiabatic (incoherent) leakage errors in the TSWT basisduring a generic time-dependent Hamiltonian evolution.

Since the direct coupling leakage error is dominated by the off-resonantfrequency component, while the non-adiabatic leakage errors aredominated by the on-resonant frequency components, the first and secondleakage bounds may be combined in the universal cost function to providean upper bound for all sources of leakage errors induced by bothoff-resonant and on-resonant leakage contributions.

The universal cost function provides increased controllability of thequantum system and enables the simulation of any unitary transformationwith a single continuous control over the Hamiltonian evolution.Consequently, many controls corresponding to a sequence of quantum gatesmay be merged into a single control to speed up quantum computationwhile improving the overall fidelity.

Example Operating Environment

FIG. 1 depicts an example system 100 for implementing arbitrary quantumgates with universal control. The example system 100 is an example of asystem implemented as classical or quantum computer programs on one ormore classical computers or quantum computing devices in one or morelocations, in which the systems, components, and techniques describedbelow can be implemented.

The system 100 includes quantum hardware 102 in data communication witha classical processor 104. The system 100 is configured to performclassical computations in combination with quantum computations usingthe classical processors 104 and the quantum hardware 102.

The quantum hardware 102 includes one or more qubits 106. The qubits 106may include qubits that can be efficiently prepared in an initial stateand operated on via application of quantum gates. The type of physicalrealizations of qubits included in the quantum hardware 102 may vary.For example, in some implementations the quantum hardware 102 mayinclude superconducting qubits, e.g., superconducting charge qubits,superconducting flux qubits or superconducting phase qubits. In otherimplementations the quantum hardware 102 may include qubits realized byspins, e.g., electron spins, nuclear spins or atomic spins. Generally,the qubits 106 may be frequency tunable.

The qubits 106 may be engineered such that each qubit energy subspace isseparated from others by a sufficiently large energy gap, as describedin more detail below with reference to FIG. 4.

The quantum hardware 102 may include a set of qubit frequency controllines, e.g., where each qubit frequency control line corresponds to anindividual qubit. The qubit frequency control lines control thefrequency of the qubits 106, e.g., where each qubit frequency controlline controls the frequency of its corresponding qubit.

The quantum hardware 102 may include one or more excitation drivelines.For convenience one driveline, e.g., driveline 108, is shown in FIG. 1,however in some implementations the quantum hardware may includemultiple drivelines, e.g., one driveline corresponding to each of thequbits 106. The one or more excitation drivelines provide excitationcontrol of the qubits 106. The one or more excitation drivelines may beconfigured to run excitation pulses (also referred to herein as controlpulses), e.g., control pulse 108, with different quantum gates atdifferent frequencies. Each qubit may be tuned towards or away fromthese frequencies on the one or more excitation drivelines.

The quantum hardware 102 may include a set of couplers. Each coupler inthe set of couplers couples a corresponding qubit to an excitationdriveline. The couplers may be any type of coupler, e.g., capacitivecouplers. In order to achieve a capacitive coupling, a microwave linemay be run adjacent to a qubit capacitor.

The quantum hardware 102 includes qubit control devices 110. The controldevices 110 include devices configured to operate on the one or morequbits 106. For example, the control devices 110 may include hardwarefor implementing quantum logic gates, e.g., control pulse generator 112that generates excitation pulses to be run on the one or more excitationdrivelines. In some implementations the control pulse generator 112 maybe a microwave control device. In addition, the control devices 112 mayinclude hardware for performing measurements on the one or more qubits106, e.g., oscillators.

The classical processor 104 is configured to perform quantum controloptimization procedures. In particular, the classical processor 104 isconfigured to design control pulses for implementing respective quantumgates. For example, the classical processor 104 may receive dataspecifying a particular unitary quantum gate or sequence of multipleunitary quantum gates, e.g., input data 114. The classical processor 104may then design control pulses that may be generated by the qubitcontrol devices 110, e.g., control pulse generator 112, and applied toone or more of the qubits 106.

The control pulses designed by the classical processor 104 may be usedto implement arbitrary unitary quantum gates with reduced leakageerrors. In some cases the control pulses designed by the classicalprocessor 104 may also be used to implement arbitrary unitary quantumgates with reduced leakage errors and one or more of increased gatefidelity or reduced total gate time.

To design such a control pulse, the classical processor 104 includes auniversal cost function generator 118 that defines a universal quantumcontrol cost function for a corresponding quantum gate or sequence ofquantum gates. The universal quantum control cost function containspenalty terms on the forms of the control pulse that will be used toimplement a unitary quantum gate. The penalty terms include a qubitleakage penalty term representing both coherent qubit leakage andincoherent qubit leakage during a time dependent Hamiltonian evolutionof the qubit or qubits that realizes the unitary quantum gate. Thepenalty terms may further include a control constraint penalty term,total gate runtime penalty term, or a gate infidelity term. The form ofsuch universal quantum control cost functions are described in moredetail below with reference to FIGS. 2-4.

The classical processor 104 adjusts parameters of time dependentHamiltonian evolution to vary a control cost according to the defineduniversal quantum control cost function to reduce leakage errors,optionally whilst also reducing gate infidelity and total gate time. Forexample, the classical processor 104 may include one or moreoptimization toolboxes that provide functions for maximizing orminimizing objectives while satisfying constraints, e.g., solvers forlinear programming, quadratic programming, nonlinear programming,constrained linear least squares, nonlinear least squares, or nonlinearequations.

The adjusted parameters resulting from such optimization processesspecify the control pulse. The classical processor 104 sends dataspecifying designed control pulses, e.g., output data 116, to thequantum hardware 102. An example process for implementing a unitaryquantum gate on one or more qubits is described in detail below withreference to FIGS. 2-4.

Programming the Hardware

FIG. 2 is a flow diagram of an example process 200 for implementing aunitary quantum gate on one or more qubits. For convenience, the process200 will be described as being performed by a system of one or moreclassical or quantum computing devices located in one or more locations.For example, the system 100 of FIG. 1 appropriately programmed inaccordance with this specification can perform the process 200.

The system designs a control pulse for the unitary quantum gate (step202). The unitary quantum gate may be a single qubit gate that operateson one qubit or a multi-qubit gate that operates on multiple qubits. Asdescribed above with reference to FIG. 1, to implement the unitaryquantum gate the control pulse may modulate a Hamiltonian characterizingthe one or more qubits according to the unitary transformation U(T)given in Equation (1) below.

U(T)=

[exp(−i∫ ₀ ^(T) Ĥ(t)dt)]  (1)

In Equation (1), T represents time-ordering, T represents total gatetime, and Ĥ(t) represents the Hamiltonian characterizing the one or morequbits, also referred to herein as the system Hamiltonian. For example,in cases where the process 200 is used to implement an arbitrarysingle-qubit gate in a superconducting qubit, e.g., a gmonsuperconducting qubit, the Hamiltonian (after application of therotating wave approximation) may be given by Equation (2) below

$\begin{matrix}{{{\hat{H}}_{g}(t)} = {{\frac{\eta}{2}{\hat{n}( {\hat{n} - 1} )}} + {{\delta(t)}\hat{n}} + {{{if}(t)}( {{\hat{a}e^{{- i}\;{\phi{(t)}}}} - {{\hat{a}}^{\dagger}e^{i\;{\phi{(t)}}}}} )}}} & (2)\end{matrix}$

In Equation (2), η represents the anharmonicity of the Josephsonjunction, {circumflex over (n)} represents the number operator,{circumflex over (α)} represents the annihilation operator, δ(t)represents qubit detuning, f (t) represents amplitude and ϕ(t)represents the phase of the control pulse, where δ(t), f (t) and ϕ(t)can be continuously modulated. The subspace spanned by two lowest energylevels of each oscillator is chosen to be the qubit logical subspaceH₂=Span{|0

, |1

}where |n

represents the Fock state with n excitations in the given mode.

When implementing the unitary quantum gate via modulation of the systemHamiltonian according to the unitary transformation U (T), leakageerrors may be introduced. The term leakage errors describes leakage ofquantum information into higher energy states outside the predefinedcomputational subspace. As described above with reference to FIG. 1,there are two sources of leakage errors—coherent leakage errorsresulting from direct coupling between a qubit's computational energystates and higher energy states and incoherent leakage errors resultingfrom violation of adiabaticity during a time dependent Hamiltonianevolution. Both types of leakage errors can further be categorized ason-resonant frequency and off-resonant frequency leakage.

Continuing the example above, a primary source of leakage errors whenmodulating the Hamiltonian Ĥ_(g)(t) can be identified by decomposing theHamiltonian Ĥ_(g)(t) into three separate componentsĤ_(g)(t)=Ĥ₀(t)+Ĥ₁(t)+Ĥ₂(t) where the first term represents thetime-independent part of the Hamiltonian:

${{{\hat{H}}_{0}(t)} = {\sum\limits_{n = 0}^{\infty}\;{\frac{\eta}{2}{\hat{n}( {\hat{n} - 1} )} n \rangle\langle n }}},$

the second term represents time—dependent Hamiltonian coupling withinthe qubit computational subspace:

${{{\hat{H}}_{1}(t)} = {{\sum\limits_{n = 0}^{\infty}\;{{\delta(t)}n n \rangle\langle n }} + {{{if}(t)}( {{ 0 \rangle\langle 1 e^{{- i}\;{\phi{(t)}}}} - { 1 \rangle\langle 0 e^{i\;{\phi{(t)}}}}} )}}},$

and the third term represents control pulse, e. g., microwave control,coupling of the qubit computatioi

$ {{ {{\hat{H}}_{2} = {{{if}(t)}{\sum\limits_{n = 1}^{\infty}\;{(  n}}}} \rangle\langle {n + 1} e^{{- i}\;{\phi{(t)}}}} - { {n + 1} \rangle\langle n e^{i\;{\phi{(t)}}}}} ),$

The primary source of leakage errors originates from the non-zeromicrowave control in the third term. This term does not conserve thetotal excitation number and couples the computational state |1

with the higher energy state |2

.

Due to the leakage errors described above, the result of the unitarytransformation U(T) may differ to the intended action U_(target) of theunitary quantum gate on the one or more qubits. The difference betweenthe intended action U_(target) of the unitary quantum gate on the one ormore qubits and the simulated unitary transformation U(T) can berepresented by the fidelity between U_(target) and U(T), where a largerfidelity indicates that the unitary quantum gate realized via theunitary transformation is more truthful to an ideal realization of theunitary quantum gate. The fidelity is given by Equation (3) below.

$\begin{matrix}{{F( {U(T)} )} = {\frac{1}{2^{2}}{{{Tr}( {{U^{\dagger}(T)}U_{target}} }^{2}}}} & (3)\end{matrix}$

The system defines a universal quantum control cost function (step 202a). The universal quantum control cost function contains penalty termson the forms of the control pulse. Such penalty terms provide the systemwith increased controllability of the one or more qubits and theimplementation of the unitary quantum gate whilst simplifying thequantum control optimization procedure, since minimizing a scalarfunction is computationally less demanding than directly optimizing ahigh dimensional unitary evolution governed by the Schrödinger equationof a time-dependent quantum system and its environment.

To suppress the total leakage errors that cause quantum information tobe lost to the environment, the universal quantum control cost functionincludes a qubit leakage penalty term L_(TSWTLB) that represents bothcoherent qubit leakage and incoherent qubit leakage during the timedependent Hamiltonian evolution that realizes the unitary quantum gate.The specific form of the qubit leakage penalty term L_(TSWTLB) isdescribed in detail below with reference to FIG. 3.

To conveniently prepare and measure qubits in the computational basis atthe beginning and the end of each Hamiltonian evolution, it is requiredthat the term representing the time-dependent Hamiltonian couplingwithin the qubit computational subspace and the term representingcontrol pulse coupling of the qubit computational subspace with thehigher energy subspace, e.g., in the case of the gmon Hamiltonian theterms Ĥ₁(t) and Ĥ₂(t), vanish at both boundaries, such that the belowdescribed TSWT transformed basis and original qubit basis coincide. Sucha control constraint may be enforced by adding a boundary controlconstraint penalty term to the total cost function. For example, in thecase of the gmon Hamiltonian given above in Equation (2), the system maydefine the universal quantum control cost function as including theboundary control constraint penalty term[δ(t)²|_(t=0.T)+f(t)²|_(t=0.T)].

To reduce the total unitary quantum gate runtime T—a desirable propertyfor near-term quantum devices—the universal quantum control costfunction may further include a total runtime penalty term.

To reduce gate infidelity—the difference between the actual gatefidelity from its maximum value 1—the universal quantum control costfunction may further include a gate fidelity penalty term 1-F(U(T)).

Continuing the example of the gmon Hamiltonian above, the totaluniversal quantum cost function defined by the system may be given byEquation (4) below.

C(α, β, γ, k)=α[1−F(U(T)]+βL _(TSMTLB)+γ[δ(t)²|_(t=0.T) +f(t)²|_(t=0.T)]+kT   (4)

In Equation (4), α penalizes the gate infidelity, β penalizes allsources of leakage error from the leakage bound L_(TSMTLB) (described inmore detail below with reference to FIG. 3), γ penalizes the violationof zero-value boundary constraint and K penalizes long gate time.

The system adjusts the control over parameters of the time dependentHamiltonian evolution, e.g., by tuning microwave pulse amplitude andphase or the flux biases across a Josephson junction, to vary a controlcost according to the defined universal quantum control cost functionsuch that leakage errors are reduced (step 202 b). For example, thesystem may apply optimization techniques, e.g., those found inconventional optimization toolboxes such as stochastic gradient descent,differential evolution, dynamic programming, annealing algorithms, MonteCarlo methods and other machine learning techniques, to determineoptimal parameters of the time dependent Hamiltonian evolution thatdefine a control pulse that may implement the unitary quantum gate withminimal leakage errors.

In cases where the control cost function includes a total runtimepenalty term, the system may adjust the parameters of the time dependentHamiltonian evolution to vary a control cost according to the controlcost function such that leakage errors are reduced and total gate timeis reduced. For example, the system may apply optimization techniques todetermine optimal parameters of the time dependent Hamiltonian evolutionthat define a control pulse that may implement the unitary quantum gatewith reduced leakage errors and reduced total gate time.

In cases where the control cost function includes a gate infidelitypenalty term, the system may adjust the parameters of the time dependentHamiltonian evolution to vary a control cost according to the controlcost function such that leakage errors are reduced and gate infidelityis reduced (gate fidelity is increased). For example, the system mayapply optimization techniques to determine optimal parameters of thetime dependent Hamiltonian evolution that define a control pulse thatmay implement the unitary quantum gate with reduced leakage errors andincreased gate fidelity.

In some cases the system may apply optimization techniques to determineoptimal parameters of the time dependent Hamiltonian evolution thatdefine a control pulse that may implement the unitary quantum gate withreduced leakage errors, reduced gate infidelity, and reduced total gatetime.

The system generates the control pulse using the adjusted parameters(step 204). Generating the control pulse may include converting thedesired control pulse over Hamiltonians to electric/magnetic signalsthat control electronics that are applied to the quantum system.

The system applies the control pulse to the one or more qubits toimplement the unitary quantum gate (step 206).

By replacing hard constraints on controls of system Hamiltonians withsoft constraints on the total leakage errors, control boundaryconditions, total gate time and gate infidelity in the form ofcorresponding penalty terms of a universal cost function for quantumcontrol optimization, the controllability of the qubits is improved. Inaddition, the process 200 allows for any unitary transformation to besimulated with a single continuous control over the Hamiltonianevolution. Consequently, in some implementations the system may mergemany controls for a sequence of quantum gates into a single control tospeed up a quantum computation being performed by the system whileimproving the overall fidelity.

Penalizing Qubit Leakage

FIG. 3 is a flow diagram of an example process 300 for defining a qubitleakage penalty term in a universal quantum control cost function. Forconvenience, the process 300 will be described as being performed by asystem of one or more classical or quantum computing devices located inone or more locations. For example, the system 100 of FIG. 1appropriately programmed in accordance with this specification canperform the process 300.

The system generates a block-off-diagonal component of an effectiveHamiltonian with direct coupling leakage errors suppressed to any givenorder (step 302). Generating a block-off-diagonal component

_(od) of an effective Hamiltonian with direct coupling leakage errorssuppressed to the given order is described in more detail below withreference to FIG. 4.

The system determines a coherent qubit leakage penalty sub-termL_(direct) using the generated block off-diagonal component of theeffective Hamiltonian (step 304). Coherent qubit leakage is caused bythe block-off-diagonal component of the effective Hamiltonian, since theblock-off-diagonal component of the effective Hamiltonian directlycouples the qubit computational energy subspace to the higher energysubspace. Therefore, to determine the coherent qubit leakage penaltysub-term L_(direct), the system evaluates the population thattransitions out of the qubit computational subspace due to the directcouplings ∥

_(od)∥/∥

_(od)∥«Δ with Δ representing the energy gap between the two lowestenergy eigenstates by considering the interacting picture with quantumstate basis |ψ(t)

_(t)=U_(d) ⁻¹(t)|ψ(0)

that relates to the Schrödinger picture basis initial state |ψ(0)

by a block diagonal Hamiltonian evolution U_(d)(t)=

[

], where

represents the time-ordering. The Schrödinger equation in theinteracting picture is given by Equation (5) below.

$\begin{matrix}{ {{{i\frac{d}{dt} {\psi(t)} \rangle_{I}} = {{U_{d}^{- 1}(t)}{{\hat{\mathbb{H}}}_{od}(t)}{U_{d}(t)} {\psi(0)} \rangle_{I}}},{\psi(t)}} \rangle_{I} \approx {\lbrack {I - {i{\int\limits_{0}^{t}{{U_{d}^{- 1}(\tau)}{{\hat{\mathbb{H}}}_{od}(\tau)}\mspace{14mu}{U_{d}(\tau)}d\;\tau}}}} \rbrack {\psi(0)} \rangle}} & (5)\end{matrix}$

In Equation (5), the initial condition |ψ(0)

_(I)=|ψ(0)

_(t) is used and a first order Taylor expansion of the time orderedintegration is performed since ∥

_(od)∥ is of the order 0 (∈³/Δ²). Since U_(d)(t) preserves thecomputational subspace, the system determines the coherent qubit leakagepenalty sub-term L_(direct) by determining a sum of the amplitudes ofall excited states outside the computational energy subspace due to theblock-off-diagonal component of the effective Hamiltonian, as givenbelow in Equation (6).

$\begin{matrix}{{{L_{direct}(t)} = {\sum\limits_{{\alpha \neq 0},{m \in \Omega_{\alpha}}} \langle {{m(t)}❘{{( {{U_{d}(t)}❘{\psi(0)}} \rangle - {U_{d}(t)}}❘{\psi(t)}}} \rangle_{I} )}}} & (6)\end{matrix}$

The system bounds the determined coherent qubit leakage penalty sub-termL_(direct) by inserting Equation (5) into Equation (6), using thepropagator relation U_(d)(t) U_(d) ⁻¹(τ)=U_(d)(t, τ), and approximatingthe diagonal unitary evolution with its dominant parts given by thetime-independent energy gap. The system then bounds L_(direct) using theFrobenius norm of the block off-diagonal component of the effectiveHamiltonian under the off-resonant assumption. The bounded coherentqubit leakage penalty sub-term is given below in Equation (7).

$\begin{matrix}\begin{matrix}{{L_{direct}(t)} =} & {{\sum\limits_{{\alpha \neq 0},{m \in \Omega_{\alpha}}} \langle {{m(t)}❘{{( {{U_{d}(t)}❘{\psi(0)}} \rangle - {U_{d}(t)}}❘{\psi(t)}}} \rangle_{I} )}} \\{\overset{2}{=}} & {\sum\limits_{{\alpha \neq 0},{m \in \Omega_{\alpha}}}{\langle {{m(t)}{{\int_{0}^{t}{{U_{d}( {t,\tau} )}{{\hat{\mathbb{H}}}_{od}(\tau)}{U_{d}( {\tau,t} )}d\;\tau}}}{\psi(t)}} \rangle }} \\{\overset{3}{\approx}} & {\sum\limits_{{\alpha \neq 0},{m \in \Omega_{\alpha}}}{\langle {{m(t)}{{\int_{0}^{t}{e^{{- i}\;{\Delta_{\alpha}{({t - \tau})}}}{{\hat{\mathbb{H}}}_{od}(\tau)}d\;\tau}}}{\psi(t)}} \rangle }} \\{\leq} & {\frac{{{\hat{\mathbb{H}}}_{od}(0)}}{\Delta(0)} + \frac{{{\hat{\mathbb{H}}}_{od}(t)}}{\Delta(t)}}\end{matrix} & (7)\end{matrix}$

Inserting the block-off-diagonal Hamiltonian after the given order TSWTdescribed below with reference to FIG. 4 into Equation (8) provides aleakage bound that depends on both the first and second derivative ofthe system Hamiltonian.

The system determines an incoherent qubit leakage penalty sub-termL_(non-adiabatic) using the generated block off-diagonal component ofthe effective Hamiltonian (step 306). Incoherent qubit leakage is causedby violation of adiabaticity during time-dependent Hamiltonianevolution, where ∥

_(od)∥/∥

_(od)∥˜Δ. To determine the incoherent qubit leakage penalty sub-termL_(non-adiabatic)) the system generalizes the applicability of theadiabatic theorem from pure states to energy subspaces, enablingtime-dependent Hamiltonian evolution to occur within and betweendifferent energy subspaces. The system then determines the incoherentqubit leakage penalty sub-term using the generalized adiabatic theorem.

The generalized adiabatic theorem may be formulated as follows. Let Ĥ(s)be a twice differentiable Hamiltonian parameterized by a unit-freere-scaled time s ∈ [0, 1] comprising three parts: Ĥ(s)+H₀+

_(d)(s)+

_(od)(s), , where H₀=Σ_(α=0) ^(∞)Σ_(m∈Ω) _(α) E_(α)|m

m| ensures sufficiently large constant energy gap between the lowestenergy subspace Ω₀ and other higher energy subspaces,

_(d)(s)is a time dependent term accounting for couplings within eachnon-degenerate subspace Ω_(α)and

_(od)(s) accounts for the coupling between different subspaces. Theseparation between the energy gap and inter/intra-subspace coupling isgiven by Δ=min/α|E_(α)-E₀|»|

_(d)(s)|˜|

_(od)(s)|˜δ. Let |ϕ₀(s)

=Σ_(m∈Ω) ₀ α_(m)(s)|m

be an instantaneous eigenstate in the lowest energy subspace Ω₀ atphysical time sT. Let |ψ(s)

be the state evolved from the same initial state |ϕ₀(0)

at time s=0 under the total Hamiltonian Ĥ(s)) to time s. The followinginequality then bounds the difference between these two states at thefinal time T:

$L_{{non}\text{-}{adiabatic}} \leq {\frac{1}{T}\lbrack {{\frac{1}{\Delta^{2}(s)}( {{\frac{d{{\hat{\mathbb{H}}}_{od}(s)}}{ds}} + {T{\lbrack {{{\hat{\mathbb{H}}}_{d}(s)},{{\hat{\mathbb{H}}}_{od}(s)}} \rbrack }}} )_{s = 1}} + {\frac{1}{\Delta^{2}(s)}( {{\frac{d{{\hat{\mathbb{H}}}_{od}(s)}}{ds}} + {T{\lbrack {{{\hat{\mathbb{H}}}_{d}(s)},{{\hat{\mathbb{H}}}_{od}(s)}} \rbrack }}} )_{s = 0}}} \rbrack}$

${{+ {\int_{0}^{1}{\frac{5}{\Delta^{3}(s)}( {{\frac{d{{\hat{\mathbb{H}}}_{od}(s)}}{ds}} + {T{\lbrack {{{\hat{\mathbb{H}}}_{d}(s)},{{\hat{\mathbb{H}}}_{od}(s)}} \rbrack }}} )^{2}{ds}}}} + {\int_{0}^{1}{\frac{1}{\Delta^{2}(s)}( {{T{\lbrack {{{\hat{\mathbb{H}}}_{d}(s)},\lbrack {{{\hat{\mathbb{H}}}_{d}(s)},{{\hat{\mathbb{H}}}_{od}(s)}} \rbrack} \rbrack }} + {2{\lbrack {{{\hat{\mathbb{H}}}_{d}(s)},\frac{d{{\hat{\mathbb{H}}}_{od}(s)}}{ds}} \rbrack }} + {2{\lbrack {\frac{d{{\hat{\mathbb{H}}}_{d}(s)}}{ds},{{\hat{\mathbb{H}}}_{od}(s)}} \rbrack }} + {\frac{1}{T}{\frac{d^{2}{{\hat{\mathbb{H}}}_{od}(s)}}{{ds}^{2}}}}} ){ds}}}},$

The dominant contribution to the incoherent qubit leakage penaltysub-term L_(non-adiabatic), is given by Equation (8) below

$\begin{matrix}{L_{{non}\text{-}{adiabatic}} = {\int_{0}^{1}{\frac{1}{\Delta^{2}(s)}\frac{1}{T}{\frac{d^{2}{{\hat{\mathbb{H}}}_{od}(s)}}{{ds}^{2}}}{ds}}}} & (8)\end{matrix}$

Since the coherent qubit leakage penalty sub-term L_(direct) dominatesin an off-resonant frequency regime ∥

_(od)∥/∥

_(od)∥«Δ and the incoherent qubit leakage penalty sub-termL_(non-adiabatic) dominates in an on-resonant frequency regime ∥

_(od)∥/∥

_(od)∥˜Δ, the system defines the qubit leakage penalty term L_(TSWTLB)as the sum of the coherent qubit leakage penalty sub-term L_(direct) andthe incoherent qubit leakage penalty sub-term L_(non-adiabatic) (step308). The qubit leakage penalty term L_(TSWTLB) is given in Equation (9)below.

$\begin{matrix}{L_{TSWTLB} = {{\int_{0}^{1}{\frac{1}{\Delta^{2}(s)}\frac{1}{T}{\frac{d^{2}{{\hat{\mathbb{H}}}_{od}(s)}}{{ds}^{2}}}{ds}}} + \frac{{{\hat{\mathbb{H}}}_{od}(0)}}{\Delta(0)} + \frac{{{\hat{\mathbb{H}}}_{od}(T)}}{\Delta(T)}}} & (9)\end{matrix}$

The leakage penalty term L_(TSWTLB) enables hard constraints on theforms of system Hamiltonian to be relaxed to soft penalty terms in theuniversal quantum control cost function. Directly optimizing theuniversal quantum control cost function allows for the reduction ofleakage errors and total gate time whilst increasing gate fidelitywithout sacrificing the controllability of the quantum system.

Generalization of the Time-dependent Schrieffer-Wolff Transformation

FIG. 4 is a flow diagram of an example process 400 generalizing thetime-dependent Schrieffer-Wolff transformation to suppress directcoupling qubit leakage to any given order. For convenience, the process400 will be described as being performed by a system of one or moreclassical or quantum computing devices located in one or more locations.For example, the system 100 of FIG. 1 appropriately programmed inaccordance with this specification can perform the process 400.

The system separates a Hamiltonian characterizing the one or more qubitsinto multiple components (step 402). The multiple components include afirst component representing time-dependent coupling between differentqubit energy subspaces, wherein the Hamiltonian is expressed in terms ofmultiple basis states. The multiple components may further include asecond component representing a time-independent part of the Hamiltonianand a third component representing time-dependent coupling within eachqubit energy subspace. For example, the Hamiltonian given above inEquation (2) may be separated into the below three components:

$\begin{matrix}{{{{\hat{H}(t)} = {{\hat{H}}_{0} + {{\hat{H}}_{1}(t)} + {{\hat{H}}_{2}(t)}}},{{\hat{H}}_{0} = {\sum\limits_{\alpha}{\sum\limits_{m \in \Omega_{\alpha}}{E_{1} m \rangle\langle m }}}},{{{\hat{H}}_{1}(t)} = {\sum\limits_{\alpha}{\sum\limits_{m \in \Omega_{\alpha}}{\langle {m{{{\hat{H}}_{1}^{\alpha}(t)}}m^{\prime}} \rangle m \rangle\langle m^{\prime} }}}}}{{{\hat{H}}_{2}(t)} = {\sum\limits_{\alpha \neq \alpha^{\prime}}{\sum\limits_{{m \in \Omega_{\alpha}},,{m^{\prime} \in \Omega_{\alpha^{\prime}}}}{\langle {m{{{\hat{H}}_{2}^{\alpha,\alpha^{\prime}}(t)}}m^{\prime}} \rangle {\alpha,m} \rangle\langle {{{\alpha^{\prime},m^{\prime}}},} }}}}} & (10)\end{matrix}$

where Ĥ₀(t) represents the time-independent part of the Hamiltonian,Ĥ₁(t) represents the time-dependent coupling within each qubit energysubspace, referred to herein as the “block-diagonal term,” and Ĥ₂(t)represents the time-dependent coupling between different qubit energysubspaces, referred to herein as the “block-off-diagonal term.” InEquation (10), Ω_(α) with α ∈ {0, 1, 2, . . . } represents differentenergy subspaces and |m

represents an orthogonal basis state spanning each same energy subspace.It is assumed that the magnitudes of each of the multiple componentsobey:

Δ=min_(α≠0)|E _(α) −E ₀|»|

m|Ĥ ₁ ^(α)(t)|m′

|˜|

m|Ĥ ₂ ^(α,α′)(t)|ι

|∈

for all m, m′ ∈0Ω_(α), ι ∈ Ω_(α), α≠α′ through our the time-dependentHamiltonian evolution, where Δ and ∈ represent the different energyscales satisfying ∈/Δ«1. Under this assumption, each qubit energysubspace Ω_(α) is separated from others by a sufficiently large energygap that is substantially larger than the intra-subspace couplings.

The system defines an effective Hamiltonian

by performing a Schrieffer-Wolff transformation to rotate theHamiltonian basis state from |ψ

to e^(−ŝ)|ψ

(step 404). In this rotated basis, the effective Hamiltonian is given bythe below Equation (11).

$\begin{matrix}{{{i\frac{d}{dt} \overset{\sim}{\psi} \rangle} = {{\hat{\mathbb{H}} \overset{\sim}{\psi} \rangle} = {{i\frac{{de}^{- \hat{S}}}{dt}e^{\hat{S}} \overset{\sim}{\psi} \rangle} + {{ie}^{- \hat{S}}\frac{d}{dt} \psi \rangle}}}},{\hat{\mathbb{H}} = {{{- i}{\sum\limits_{j = 0}^{\infty}\;{\frac{1}{( {j + 1} )!}\lbrack {\overset{.}{\hat{S}},\hat{S}} \rbrack}_{j}}} + {e^{- \hat{S}}\hat{H}e^{\hat{S}}}}},} & (11)\end{matrix}$

In Equation (11), the anti-Hermitian rotation operator Ŝ(t) containsnon-zero terms between different energy subspaces and is thereforeblock-diagonal.

The system determines a given order solution of the rotation operatorŜ(t) to generate a block-off-diagonal component

_(od) of the effective Hamiltonian

with direct coupling leakage errors suppressed to the given order (step406). In particular, the system determines a perturbative solution ofthe rotation operator Ŝ(t)=∈Ŝ₁(t)+∈Ŝ₂(t)+. . . + ∈^(n)Ŝ_(n)(t) thatblock diagonalizes the Hamiltonian Ĥ(t) such that the effectiveHamiltonian's block-off-diagonal terms are suppressed to an order of 0(∈^(n+1)/Δ^(n)) for the n-th order perturbative solution of Ŝ(t). Afterthe second order TSWT the block-off diagonal component

_(od) of the effective Hamiltonian with coupling suppressed to the orderof 0 (∈⁴/Δ³) is given by Equation (12) below.

$\begin{matrix}{( {\hat{\mathbb{H}}}_{od} )^{\alpha,\alpha^{\prime}} = {\frac{{( {\hat{H}}_{1}^{\alpha} )^{2}{\hat{H}}_{2}^{\alpha,\alpha^{\prime}}} - {2{\hat{H}}_{1}^{\alpha}{\hat{H}}_{2}^{\alpha,\alpha^{\prime}}{\hat{H}}_{1}^{\alpha^{\prime}}} + {{\hat{H}}_{2}^{\alpha,\alpha^{\prime}}( {\hat{H}}_{1}^{\alpha^{\prime}} )}^{2}}{( {E_{\alpha^{\prime}} - E_{\alpha}} )^{2}} + {\frac{2}{3( {E_{\alpha^{\prime}} - E_{\alpha}} )}{\sum\limits_{\gamma}\lbrack {\frac{{\hat{H}}_{2}^{\alpha,{\alpha^{\prime}{\hat{H}}_{2}^{\alpha^{\prime},\gamma}}}{\hat{H}}_{2}^{\gamma,\alpha^{\prime}}}{E_{\alpha^{\prime}} - E_{\gamma}} - \frac{{\hat{H}}_{2}^{\alpha,\gamma}{\hat{H}}_{2}^{\gamma,\alpha}{\hat{H}}_{2}^{\alpha,\alpha^{\prime}}}{E_{\alpha} - E_{\gamma}}} \rbrack}} - {i\lbrack \frac{{{\overset{.}{\hat{H}}}_{1}^{\alpha}{\hat{H}}_{2}^{\alpha,\alpha^{\prime}}} + {2{\hat{H}}_{1}^{\alpha}{\overset{.}{\hat{H}}}_{2}^{\alpha,\alpha^{\prime}}} - {2{\overset{.}{\hat{H}}}_{2}^{\alpha,\alpha^{\prime}}{\hat{H}}_{1}^{\alpha^{\prime}}} - {{\hat{H}}_{2}^{\alpha,\alpha^{\prime}}{\overset{.}{\hat{\underset{\_}{H}}}}_{1}^{\alpha^{\prime}}}}{( {E_{\alpha^{\prime}} - E_{\alpha}} )^{2}} \rbrack} - \frac{{\overset{\sim}{\hat{H}}}_{2}^{\alpha,\alpha^{\prime}}}{( {E_{\alpha^{\prime}} - E_{\alpha}} )^{2}} + {{O( \frac{\epsilon^{4}}{\Delta^{3}} )}.}}} & (12)\end{matrix}$

Implementations of the digital and/or quantum subject matter and thedigital functional operations and quantum operations described in thisspecification can be implemented in digital electronic circuitry,suitable quantum circuitry or, more generally, quantum computationalsystems, in tangibly-embodied digital and/or quantum computer softwareor firmware, in digital and/or quantum computer hardware, including thestructures disclosed in this specification and their structuralequivalents, or in combinations of one or more of them. The term“quantum computational systems” may include, but is not limited to,quantum computers, quantum information processing systems, quantumcryptography systems, or quantum simulators.

Implementations of the digital and/or quantum subject matter describedin this specification can be implemented as one or more digital and/orquantum computer programs, i.e., one or more modules of digital and/orquantum computer program instructions encoded on a tangiblenon-transitory storage medium for execution by, or to control theoperation of, data processing apparatus. The digital and/or quantumcomputer storage medium can be a machine-readable storage device, amachine-readable storage substrate, a random or serial access memorydevice, one or more qubits, or a combination of one or more of them.Alternatively or in addition, the program instructions can be encoded onan artificially-generated propagated signal that is capable of encodingdigital and/or quantum information, e.g., a machine-generatedelectrical, optical, or electromagnetic signal, that is generated toencode digital and/or quantum information for transmission to suitablereceiver apparatus for execution by a data processing apparatus.

The terms quantum information and quantum data refer to information ordata that is carried by, held or stored in quantum systems, where thesmallest non-trivial system is a qubit, i.e., a system that defines theunit of quantum information. It is understood that the term “qubit”encompasses all quantum systems that may be suitably approximated as atwo-level system in the corresponding context. Such quantum systems mayinclude multi-level systems, e.g., with two or more levels. By way ofexample, such systems can include atoms, electrons, photons, ions orsuperconducting qubits. In many implementations the computational basisstates are identified with the ground and first excited states, howeverit is understood that other setups where the computational states areidentified with higher level excited states are possible. The term “dataprocessing apparatus” refers to digital and/or quantum data processinghardware and encompasses all kinds of apparatus, devices, and machinesfor processing digital and/or quantum data, including by way of examplea programmable digital processor, a programmable quantum processor, adigital computer, a quantum computer, multiple digital and quantumprocessors or computers, and combinations thereof. The apparatus canalso be, or further include, special purpose logic circuitry, e.g., anFPGA (field programmable gate array), an ASIC (application-specificintegrated circuit), or a quantum simulator, i.e., a quantum dataprocessing apparatus that is designed to simulate or produce informationabout a specific quantum system. In particular, a quantum simulator is aspecial purpose quantum computer that does not have the capability toperform universal quantum computation. The apparatus can optionallyinclude, in addition to hardware, code that creates an executionenvironment for digital and/or quantum computer programs, e.g., codethat constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, or a combination of one or moreof them.

A digital computer program, which may also be referred to or describedas a program, software, a software application, a module, a softwaremodule, a script, or code, can be written in any form of programminglanguage, including compiled or interpreted languages, or declarative orprocedural languages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, or other unitsuitable for use in a digital computing environment. A quantum computerprogram, which may also be referred to or described as a program,software, a software application, a module, a software module, a script,or code, can be written in any form of programming language, includingcompiled or interpreted languages, or declarative or procedurallanguages, and translated into a suitable quantum programming language,or can be written in a quantum programming language, e.g., QCL orQuipper.

A digital and/or quantum computer program may, but need not, correspondto a file in a file system. A program can be stored in a portion of afile that holds other programs or data, e.g., one or more scripts storedin a markup language document, in a single file dedicated to the programin question, or in multiple coordinated files, e.g., files that storeone or more modules, sub-programs, or portions of code. A digital and/orquantum computer program can be deployed to be executed on one digitalor one quantum computer or on multiple digital and/or quantum computersthat are located at one site or distributed across multiple sites andinterconnected by a digital and/or quantum data communication network. Aquantum data communication network is understood to be a network thatmay transmit quantum data using quantum systems, e.g. qubits. Generally,a digital data communication network cannot transmit quantum data,however a quantum data communication network may transmit both quantumdata and digital data.

The processes and logic flows described in this specification can beperformed by one or more programmable digital and/or quantum computers,operating with one or more digital and/or quantum processors, asappropriate, executing one or more digital and/or quantum computerprograms to perform functions by operating on input digital and quantumdata and generating output. The processes and logic flows can also beperformed by, and apparatus can also be implemented as, special purposelogic circuitry, e.g., an FPGA or an ASIC, or a quantum simulator, or bya combination of special purpose logic circuitry or quantum simulatorsand one or more programmed digital and/or quantum computers.

For a system of one or more digital and/or quantum computers to be“configured to” perform particular operations or actions means that thesystem has installed on it software, firmware, hardware, or acombination of them that in operation cause the system to perform theoperations or actions. For one or more digital and/or quantum computerprograms to be configured to perform particular operations or actionsmeans that the one or more programs include instructions that, whenexecuted by digital and/or quantum data processing apparatus, cause theapparatus to perform the operations or actions. A quantum computer mayreceive instructions from a digital computer that, when executed by thequantum computing apparatus, cause the apparatus to perform theoperations or actions.

Digital and/or quantum computers suitable for the execution of a digitaland/or quantum computer program can be based on general or specialpurpose digital and/or quantum processors or both, or any other kind ofcentral digital and/or quantum processing unit. Generally, a centraldigital and/or quantum processing unit will receive instructions anddigital and/or quantum data from a read-only memory, a random accessmemory, or quantum systems suitable for transmitting quantum data, e.g.photons, or combinations thereof.

The essential elements of a digital and/or quantum computer are acentral processing unit for performing or executing instructions and oneor more memory devices for storing instructions and digital and/orquantum data. The central processing unit and the memory can besupplemented by, or incorporated in, special purpose logic circuitry orquantum simulators. Generally, a digital and/or quantum computer willalso include, or be operatively coupled to receive digital and/orquantum data from or transfer digital and/or quantum data to, or both,one or more mass storage devices for storing digital and/or quantumdata, e.g., magnetic, magneto-optical disks, optical disks, or quantumsystems suitable for storing quantum information. However, a digitaland/or quantum computer need not have such devices.

Digital and/or quantum computer-readable media suitable for storingdigital and/or quantum computer program instructions and digital and/orquantum data include all forms of non-volatile digital and/or quantummemory, media and memory devices, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices; magnetic disks, e.g., internal hard disks or removable disks;magneto-optical disks; CD-ROM and DVD-ROM disks; and quantum systems,e.g., trapped atoms or electrons. It is understood that quantum memoriesare devices that can store quantum data for a long time with highfidelity and efficiency, e.g., light-matter interfaces where light isused for transmission and matter for storing and preserving the quantumfeatures of quantum data such as superposition or quantum coherence.

Control of the various systems described in this specification, orportions of them, can be implemented in a digital and/or quantumcomputer program product that includes instructions that are stored onone or more non-transitory machine-readable storage media, and that areexecutable on one or more digital and/or quantum processing devices. Thesystems described in this specification, or portions of them, can eachbe implemented as an apparatus, method, or system that may include oneor more digital and/or quantum processing devices and memory to storeexecutable instructions to perform the operations described in thisspecification.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of what may beclaimed, but rather as descriptions of features that may be specific toparticular implementations. Certain features that are described in thisspecification in the context of separate implementations can also beimplemented in combination in a single implementation. Conversely,various features that are described in the context of a singleimplementation can also be implemented in multiple implementationsseparately or in any suitable sub-combination. Moreover, althoughfeatures may be described above as acting in certain combinations andeven initially claimed as such, one or more features from a claimedcombination can in some cases be excised from the combination, and theclaimed combination may be directed to a sub-combination or variation ofa sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various system modulesand components in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

Particular implementations of the subject matter have been described.Other implementations are within the scope of the following claims. Forexample, the actions recited in the claims can be performed in adifferent order and still achieve desirable results. As one example, theprocesses depicted in the accompanying figures do not necessarilyrequire theThe system determines a coherent qubit particular ordershown, or sequential order, to achieve desirable results. In some cases,multitasking and parallel processing may be advantageous.

1. A method for implementing a unitary quantum gate on one or morequbits, the method comprising: defining a universal quantum control costfunction, wherein the control cost function comprises a total runtimepenalty term and a qubit leakage penalty term representing qubit leakageduring a time dependent Hamiltonian evolution that realizes the unitaryquantum gate; adjusting parameters of the time dependent Hamiltonianevolution to vary a control cost according to the control cost functionsuch that total quantum gate runtime and qubit leakage errors arereduced; generating a control pulse for a unitary quantum gate using theadjusted parameters; and applying the control pulse to the one or morequbits to implement the unitary quantum gate.
 2. The method of claim 1,wherein the universal control cost function further comprises a controlconstraint penalty term.
 3. The method of claim 1, wherein the universalcontrol cost function further comprises a gate fidelity penalty term. 4.The method of claim 3, wherein adjusting parameters of the timedependent Hamiltonian evolution comprises adjusting parameters of thetime dependent Hamiltonian evolution to vary the control cost accordingto the control cost function such that quantum gate fidelity isincreased.
 5. The method of claim 1, wherein defining the universalquantum control cost function comprises: generating a block-off-diagonalcomponent of an effective Hamiltonian with direct coupling leakageerrors suppressed to any given order; determining a coherent qubitleakage penalty sub-term using the generated block off-diagonalcomponent of the effective Hamiltonian; determining an incoherent qubitleakage penalty sub-term using the generated block off-diagonalcomponent of the effective Hamiltonian; and defining the qubit leakagepenalty term as the sum of the coherent qubit leakage penalty sub-termand the incoherent qubit leakage penalty sub-term of both on-resonantand off-resonant frequency components.
 6. The method of claim 5, whereingenerating the block-off-diagonal component of an effective Hamiltoniancomprises generalizing the time-dependent Schrieffer-Wolfftransformation to suppress direct coupling qubit leakage to any givenorder, comprising: separating a Hamiltonian characterizing the one ormore qubits into multiple components, the components comprising a firstcomponent representing time-dependent coupling between different qubitenergy subspaces, wherein the Hamiltonian is expressed in terms ofmultiple basis states; defining an effective Hamiltonian by performing aSchrieffer-Wolff transformation to rotate the Hamiltonian basis states,the effective Hamiltonian comprising an anti-Hermitian rotationoperator; and determining a given order solution of the rotationoperator to generate a block-off-diagonal component of the effectiveHamiltonian with direct coupling leakage errors suppressed to the givenorder, optionally wherein the given order is second order.
 7. The methodof claim 1, wherein coherent leakage comprises leakage resulting fromdirect coupling between a qubit's computational energy states and higherenergy states.
 8. The method of claim 1, wherein adjusting parameters ofthe time dependent Hamiltonian evolution to vary a control costaccording to the control cost function such that leakage errors arereduced comprises applying optimization techniques.
 9. The method ofclaim 1, wherein the one or more qubits comprise superconducting qubits.10. An apparatus for implementing a single qubit unitary quantum gate,the apparatus comprising: one or more classical processors; a quantumdevice in data communication with the one or more classical processors,wherein the quantum device comprises: one or more qubits; one or morecontrol pulse drivelines; one or more couplers, each coupler coupling acorresponding qubit to a control pulse driveline; a control pulsegenerator configured to generate control pulses on the one or moredrivelines; wherein the apparatus is configured to perform operationscomprising: defining a universal quantum control cost function, whereinthe control cost function comprises a total runtime penalty term and aqubit leakage penalty term representing qubit leakage during a timedependent Hamiltonian evolution that realizes the unitary quantum gate;adjusting parameters of the time dependent Hamiltonian evolution to varya control cost according to the control cost function such that totalquantum gate runtime and qubit leakage errors are reduced; generating acontrol pulse for the unitary quantum gate using the adjustedparameters; and applying the control pulse to the one or more qubits toimplement the unitary quantum gate.
 11. The apparatus of claim 10,wherein the universal control cost function further comprises a controlconstraint penalty term.
 12. The apparatus of claim 10, wherein theuniversal control cost function further comprises a gate fidelitypenalty term.
 13. The apparatus of claim 12, wherein adjustingparameters of the time dependent Hamiltonian evolution comprisesadjusting parameters of the time dependent Hamiltonian evolution to varythe control cost according to the control cost function such thatquantum gate fidelity is increased.
 14. The apparatus of claim 10,wherein defining the universal quantum control cost function comprises:generating a block-off-diagonal component of an effective Hamiltonianwith direct coupling leakage errors suppressed to any given order;determining a coherent qubit leakage penalty sub-term using thegenerated block off-diagonal component of the effective Hamiltonian;determining an incoherent qubit leakage penalty sub-term using thegenerated block off-diagonal component of the effective Hamiltonian; anddefining the qubit leakage penalty term as the sum of the coherent qubitleakage penalty sub-term and the incoherent qubit leakage penaltysub-term of both on-resonant and off-resonant frequency components. 15.The apparatus of claim 14, wherein generating the block-off-diagonalcomponent of an effective Hamiltonian comprises generalizing thetime-dependent Schrieffer-Wolff transformation to suppress directcoupling qubit leakage to any given order, comprising: separating aHamiltonian characterizing the one or more qubits into multiplecomponents, the components comprising a first component representingtime-dependent coupling between different qubit energy subspaces,wherein the Hamiltonian is expressed in terms of multiple basis states;defining an effective Hamiltonian by performing a Schrieffer-Wolfftransformation to rotate the Hamiltonian basis states, the effectiveHamiltonian comprising an anti-Hermitian rotation operator; anddetermining a given order solution of the rotation operator to generatea block-off-diagonal component of the effective Hamiltonian with directcoupling leakage errors suppressed to the given order, optionallywherein the given order is second order.
 16. The apparatus of claim 10,wherein coherent leakage comprises leakage resulting from directcoupling between a qubit's computational energy states and higher energystates.
 17. The apparatus of claim 10, wherein adjusting parameters ofthe time dependent Hamiltonian evolution to vary a control costaccording to the control cost function such that leakage errors arereduced comprises applying optimization techniques.
 18. The apparatus ofclaim 10, wherein the one or more qubits comprise superconductingqubits.